1. Field of the Invention
The variable capacitance audio cable relates to the transmission of audio information from a source (typically a guitar or musical instrument) to a sink (typically an audio amplifier or other audio processing equipment).
2. Description of the Prior Art
An audio cable is intended to connect a source of audio information to a consumer or sink for that information, typically an amplifier, which amplifier may have a connected loudspeaker, or may be a front end for other processing such as a computer. A special case of this arrangement occurs when a musical instrument containing one or more sound pickups is connected to an audio amplifier. This case is of interest because of the nature of the sound pickups, which in the majority have a passive construction. Such passive sound pickups (or simply pickups) have a high output impedance in the general range of a few thousand to twenty five thousand ohms, and as such are susceptible to the effects of the interelectrode capacitance of the audio cable used to connect the instrument to the amplifier. If, however, the pickup or audio source has a low output impedance (in the case of a pre-amplified pickup), then the capacitance of the audio cable is of little consequence and causes practically no audible degradation in the frequency response of the source audio information.
Well known in the prior art is an electronic technique for reducing the capacitance of an audio cable, described presently. Audio cables typically have coaxial construction, with a center conductor (or wire), a dielectric layer, a co-axial, cylindrical shield made of spiraled or braided conductors, or a metal foil layer, or both, and an overall insulating layer. The capacitance between the center conductor and the shield ranges generally from 10 picofarads/foot (pf/ft) to 60 pf/ft or more, depending on the specific geometries and dielectric involved.
To reduce the interelectrode capacitance of the cable, a second shield layer is interposed between the original shield and the center conductor, and insulated from both. This second shield is driven with a buffered version of the signal on the center conductor, produced with a noninverting unity gain voltage amplifier. The net result is that the AC voltage between the center conductor and the driven second shield is zero, and thus zero AC current flows through the cylindrical capacitor structure of the audio cable from center conductor to outer shield, making it appear to the audio source (or pickup) as if the cable has zero capacitance. The outer shield of the cable provides overall shielding and a return (also called ‘ground’) path for current flowing in the center conductor. The driven shield conductor, being driven with a low output impedance voltage amplifier, also acts as an additional shield against noise sources that might otherwise affect the signal on the center conductor of the cable.
Note that the noninverting unity gain amplifier effectively has its output and input coupled together through the capacitance in the audio cable. While technically a unity gain amplifier would oscillate under these conditions, in reality a unity gain operational amplifier or equivalent sees a loop gain slightly less than one due to imperfections in the system, such as conductor resistance and a finite amplifier output impedance, so that the system does not oscillate. Please note that while the term “unity gain” is used herein, it should always be understood that the loop gain must be less than one to ensure no oscillations will occur.
This capacitance reduction technique is common in the prior art and is used by electrical engineers to mitigate the effects of capacitance when connecting high impedance sensors through cables to measurement equipment. This technique is even used in integrated circuit structures to reduce the effects of interelectrode capacitance.
For example, Bonin (U.S. Pat. No. 7,277,267, Oct. 2, 2007) states, “Avoiding this parasitic capacitance was done by feeding a unity gain buffered replica of the pickup electrode signal to the driven shields.”
Vranish (U.S. Pat. No. 6,847,354, Jan. 25, 2005) states, “Thus, the system performs as a multi-pixel sensor array in which all pixels and the driven shield are at the same voltage and at all times in phase.” This invention deals with interactive displays.
Kumada, et al. (U.S. Pat. No. 6,681,630, Jan. 27, 2004) discloses a vibrating gyroscope that uses the driven shield approach in its measurements, and states, “According to the above-explained structure, shields 8a and 8b are biased or driven with an electric potential which is the same as the electric potential of the detection signals transmitted via the wirings 7a and 7b. As a result, the surrounding of the wirings 7a and 7b are kept at an electric potential which is the same as the detection signals transmitted via the wirings 7a and 7b, thereby preventing a parasitic capacitances from being produced.”
There are many more examples in the prior art of such driven shield designs, including: Olson (U.S. Pat. No. 6,597,164, Jul. 22, 2003), Richardson, et al. (U.S. Pat. No. 4,058,765, Nov. 15, 1977), Zimmerman, et al. (U.S. Pat. No. 6,542,717, Apr. 1, 2003), Stanley, et al. (U.S. Pat. No. 6,825,765, Nov. 30, 2004), Reinbold, et al. (U.S. Pat. No. 6,033,370, Mar. 7, 2000), Brenner, et al. (U.S. Pat. No. 5,973,415, Oct. 26, 1999), Vranish (U.S. Pat. No. 5,726,581, Mar. 10, 1998), Satterwhite (U.S. Pat. No. 5,519,329, May 21, 1996), Vranish (U.S. Pat. No. 5,442,347, Aug. 15, 1995), Vranish (U.S. Pat. No. 5,373,245, Dec. 13, 1994), Zweifel (U.S. Pat. No. 5,365,783, Nov. 22, 1994), Pangerl (U.S. Pat. No. 5,347,867, Sep. 20, 1994), Vranish, et al. (U.S. Pat. No. 5,166,679, Nov. 24, 1992), and Dunseath, Jr. (U.S. Pat. No. 4,751,471, Jun. 14, 1988). For brevity, the quotations have not been included here.
All of these patents describe the driven shield technique that includes, and includes only, a driving of a shield conductor with a signal that is a one-to-one replica in amplitude, frequency, and phase of the signal appearing on the structure or conductor being shielded. These applications are diverse and cover many areas of invention.
However, the prior art does not disclose, and, in teaching only a unity gain buffer amplifier, ignores the use of a non-unity gain transfer function in the circuitry that drives the driven shield. The important and unexpected benefits of this new configuration are described following.
When an audio cable is connected to a sound pickup, that cable affects the frequency response of the signal conveyed to the amplifier. If the pickup has a resistive (non-complex) output impedance, the output impedance and the cable capacitance work together to lowpass filter the audio signal. If the pickup has an inductive character, as with pickups used on many stringed musical instruments, then the cable capacitance, pickup resistance, pickup shunt capacitance, and pickup inductance work together to create a second order lowpass filter that may have a resonant peak near the cutoff frequency, depending on the damping factor of the transfer function. The specific values of pickup impedance, cable capacitance, and amplifier input impedance determine the particular frequency response that results, but the cutoff frequency is generally in the audio frequency range between a few hundred and a few thousand Hertz.
Players of stringed musical instruments that use inductive magnetic pickups in particular are keenly aware of the affect of cable capacitance on the tone of the audio being produced by their instruments. Such magnetic pickups are used to sense the vibrations of ferromagnetic strings on the instrument. Musicians experiment at great length with various brands and configurations of audio cables, pickups and amplifiers to obtain a sound that enhances their performance.
Until now, the cables available to musicians have mostly been passive in nature (containing no active electronics), having a cable capacitance fixed by cable geometry. Musicians are in the main not educated in technical matters and may not fully understand the concept of cable capacitance, and thus only understand the rudiments of this phenomenon, that is, a longer cable reduces high frequencies more than a shorter cable of the same type. A musician must experiment by purchasing individual audio cables and evaluating the effects on the sound of the instrument, and this can be a time consuming and expensive proposition.
Many musical instruments with sound pickups have integrated tone controls which simply add more capacitance on the pickup signal wire, thus reducing high frequency response. However, there is no way to selectably or variably reduce the capacitance of the pickup-cable-amplifier system below that of the audio cable, using prior art inventions.
It is possible to eliminate the effects of cable capacitance entirely by installing a buffer amplifier in the instrument or cable. Such amplifiers typically have a high impedance input (which does not appreciably load the sound pickup) and a low impedance output. The low output impedance raises the cutoff frequency of the lowpass filter formed by the amplifier output impedance and shunt cable capacitance. However, due to the impedance transformation of the amplifier, this buffer amplifier changes the dynamic feel of the instrument to the musician, who interacts not only with the instrument, but also with the amplifier and loudspeakers during a performance, sometimes incurring intentional oscilliatory feedback between the instrument and loudspeakers. For this reason, most instrumentalists avoid actively amplified instruments and cables.
An advantage of the driven shield approach to cable capacitance elimination is that the galvanic connection between the instrument and the amplifier is not disturbed. The only net effect is an elimination of cable capacitance, and not any fundamental impedance change to the instrument itself.
What is needed is a way to vary the capacitance of an audio cable, from a very low value up to the natural capacitance dictated by the geometry of the cable. Further, tailoring the frequency and phase response of the amplifier that drives the shield changes the overall response of the pickup-cable-amplifier system and presents new tonal opportunities to the musician.
Objects and Advantages of the Variable Capacitance Audio Cable
Several objects and advantages of the variable capacitance audio cable are:                1. Varying the capacitance of the audio cable allows the musician to explore tonalities heretofore not available.        2. Varying the frequency, amplitude and phase response of the amplifier driving the driven shield allows the musician to explore tonalities heretofore not available.        3. This technique has the benefit of decoupling the cable geometry from the capacitance of the cable, allowing separate optimization of physical and audio performance parameters by the cable manufacturer.        4. Elimination or reduction of the cable capacitance gives the instrument's tone control a wider range of subjective tonal variation than that had when using a standard capacitive audio cable.        5. Elimination or reduction of the cable capacitance prevents the instrument's volume control from creating a lowpass filter effect that robs the instrument of tone at low playing volumes.        6. The instrument is connected galvanically to the amplifier without intervening buffer amplifiers or impedance converters, preserving what many instrumentalists consider the ‘organic’ feel of the instrument/amplifier system.        7. The electronic circuitry used to reduce or eliminate the cable capacitance may be located at any point along the audio cable, or in the instrument, or in the amplifier to which the audio cable connects, as is convenient and economical.        